Compositor: How to crop an image into multiple versions?
This can't be done in Blender, because the image pixel dimensions will always be that of the Scene from which the Render button was pushed.

I used a File Output node to test this, saving rendered data from different Scenes, each with different render dimensions. Their dimensions were overridden by the dimensions of the main Scene.

I'll post the .blend for anyone curious. It's nothing special.



What this means is that even if you use nodes to get the scaling and cropping right, you will just end up with transparency around the image. (If scaling up your image will be clipped instead.)

This sort of task should be done with other software, such as imagemagick or Photoshop (using Actions). Here is how to do it with imagemagick using scripting.